Thermoplastic vulcanizates (TPVs) comprise finely-divided rubber particles dispersed within a thermoplastic matrix. These rubber particles are crosslinked to promote elasticity. The dispersed rubber phase is typically referred to as the discontinuous phase, and the thermoplastic phase is typically referred to as the continuous phase, although co-continuous morphologies are possible.
Thermoplastic vulcanizates may advantageously be prepared by dynamically vulcanizing a rubber with a curative agent while the rubber is being mixed with a thermoplastic resin. The usefulness of these compositions can depend on the physical properties of the compositions as well as the aesthetics of the products prepared from the compositions. Some of the important physical properties of thermoplastic vulcanizate compositions include stress at break and maximum elongation, as well as resistance to compression set. Useful aesthetic characteristics include extrusion surface roughness (ESR).
Technologically useful thermoplastic vulcanizates, which have an advantageous balance of properties, have historically been prepared from mono-modal, high molecular weight ethylene-based elastomers such as ethylene-propylene (α-olefin)-diene (EPDM) elastomers. As the skilled person appreciates, high molecular weight EPDM elastomers may possess very high viscosities, e.g., Mooney viscosity greater than 200 ML(1+4@125° C.), which results in difficulties related to the processability of these polymers. Typically, these polymers are not processable when having Mooney viscosities above about 100 ML(1+4@125° C.). As a result, the EPDM often used in the manufacture of thermoplastic vulcanizates may include extender oil. The required level of extender oil can depend on the molecular weight of the elastomer, but is usually sufficient to reduce the apparent viscosity of the oil extended EPDM to a Mooney viscosity of about 100 ML(1+4@125° C.) or below. Commercially available, mono-modal high molecular weight EPDMs, which are useful in TPVs, typically contain from about 50 to about 125 phr (parts-by-weight per 100 parts-by-weight rubber) extender oil. For example, EPDM obtained under the tradename Vistalon™ 3666 (ExxonMobil Chemical Company) has been widely used for the manufacture of technologically useful thermoplastic vulcanizates.
Many attempts to alter the rubber used in the manufacture of thermoplastic vulcanizates have been reported. For example, WO 2009/123609 is directed to thermoplastic vulcanizates prepared by employing metallocene-synthesized bimodal EPDM polymer. This bimodal EPDM has an overall Mooney viscosity ML(1+4@125° C.) of at least 30, an average branching index that is greater than 0.8, and includes less than 10 parts by weight, per 100 parts by weight rubber, of extender oil. The EPDM is bimodal to the extent that it includes a first, high-Mooney polymer fraction having a Mooney viscosity MST(5+4@200° C.) of at least 120 and a second, lower-Mooney polymer fraction having a Mooney viscosity ML(1+4@125° C.) of less than 120. Both fractions, individually, are characterized by a molecular weight distribution of less than 4.
Bimodal EPDM terpolymers that have been prepared using Zeigler Natta catalysis have also been used in thermoplastic vulcanizates. For example, WO 2008/016429 is directed to thermoplastic vulcanizates prepared by using oil-extended bimodal EPDM having an oil-extended Mooney viscosity ML(1+4@125° C.) of from 20 to 70, a molecular weight distribution between 2 and 10, and a branching index between 0.3 and 1. The EPDM is bimodal to the extent that it includes a first polymer fraction having a Mooney viscosity MST(5+4@200° C.) between 30 and 100, and a second polymer fraction having a Mooney viscosity ML(1+4@125° C.) between 10 and 120.
Other potentially relevant references include U.S. Pat. No. 7,910,637, WO 2016/076969, WO 2009/035579, WO 2003/066725, and WO 2000/26296, and EP 552945.
While attempts to replace conventional oil-extended EPDM used in thermoplastic vulcanizates have proven useful, the ability to obtain certain aesthetic properties, such as extrusion surface roughness, that are equivalent to conventional oil-extended EPDM remains a challenge.